Your reaction is not unfounded. Have a look at these:
“Subjective Value and Market prices”
“Cardinal Utility: Its Worse Than You Thought”
“Ordinal Marginal Utility and ‘Total Utility’”
“Problems with the Cost Theory of Value”
And I’m assuming you’re reading that textbook for a school class? I would strongly recommend one of these to at least go along with it (of course if you don’t have to read Mankiw, you may wish to find a better use for your time.)
Any one of the ones on the top row here. And these here. (I would also add Basic Economics to that top row as well). For a more formal introduction to real economics, check out the links here. The home study course and the Mises Academy are top notch.